1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, facsimile device and a printer, a charging device and a process unit which are installed in such devices. Moreover, the present invention relates to a rotating member production method for producing a rotating member installed in a charging device.
2. Description of the Background Art
There has been conventionally well-known an image forming apparatus in which after forming a latent image through writing with light onto a latent image supporting body which is subjected to charging evenly by a charging device, a toner as an image forming substance attached to the latent image to obtain a visible image. Furthermore, as a charging device installed in the above type of image forming apparatus, there has been known a contact roller type charging device for subjecting a latent image supporting body to charging evenly by bringing a charging roller applied with a charging bias into a contact the latent image supporting body while rotating the charging roller. There has been also known a non-contact roller type charging device for subjecting a latent image supporting body to charging evenly through discharge from a discharging portion such as a conductive rubber, while keeping a predetermined charge gap between the discharging portion and latent image supporting body in the charging roller. In comparison with the contact roller type charging device, the non-contact roller type charging device can stably maintain the charging performance, since it is difficult to fix the toner to the roller surface due to less transition of the toner from the latent image supporting body to the charging roller. For such a non-contact roller type charging device, the one described in, for example, Japanese Patent Application Laid-Open No. 2001-194868 and Japanese Patent Application Laid-Open No. 2002-55508 is known.
In the non-contact roller type charging device described in abovementioned Japanese Patent Application Laid-Open No. 2001-194868, there is used a charging roller, which is obtained by fixing a conductive rubber to a surface of a cored bar, and winding a film member with a thickness of approximately 100 [μm] around each end portion of the conductive rubber in the direction of a roller axis line. By abutting the film members on both end portions onto a drum-like photoconductor, which is a latent image supporting body, a gap of approximately 100 [μm] is kept between the photoconductor and a discharging portion which is a section on the conductive rubber, around which the film member is not wound. In such a charging roller, if the degree of hardness of the conductive rubber is reduced as the environmental temperature increases, the degree to which the film member is embedded in the conductive rubber increases, the film rubber being pressure-welded to the photoconductor. Therefore, the above-described charge gap is reduced. On the other hand, if the degree of hardness of the conductive rubber is increased as the environmental temperature decreases, the degree to which the film member is embedded in the conductive rubber decreases, thus the charge gap becomes larger. As a result, the discharging portion is brought too close to the photoconductor when the temperature becomes relatively high, whereby the toner is easily fixed to the discharging portion, or the discharging portion is brought too far from the photoconductor when the temperature becomes relatively low, thereby causing charge irregularity in the photoconductor.
In order to reduce such toner adhesion and charge irregularity, the present applicants have experimentally produced a charging roller in which is used, as a material for a discharging portion, a conductive rigid plastic instead of a flexible conductive robber. However, in this charging roller, a film member with a thickness of approximately 50 [μm], which is used as an abutting member to be abutted onto a photoconductor, falls off the surface of the discharging portion, which is made of the rigid plastic, due to friction with the photoconductor. For this reason, this charging roller could not provide durability to be able to respond to actual use.
In order to resolve such a defect related to durability, the charging roller which is proposed in abovementioned Japanese Patent Application Laid-Open No. 2002-55508 is a charging roller in which a concave portion extending in the direction of the roller circumference is provided on each end portion in the direction of a roller axis line in a discharging portion, and a heat shrinkable tube is thermally shrunk within the concave portions so that it slightly protrudes from upper ends of the concave portions, and is abutted onto a photoconductor. In this configuration, by embedding the heat shrinkable tube in the concave portions and fixing it firmly to the discharging portion, the tube as an abutting member can be prevented from falling off, and preferable durability to be able to respond to actual use can be achieved.
However, in this charging roller, thickness deviation in the tube periphery after the tube is shrunk is large, thus a charge gap per rotation of the charging roller fluctuates significantly. In relatively high environmental temperature, when a thin part of the tube was abutted onto the photoconductor, the charge gap became extremely small, and the toner was fixed easily to the discharging portion. Moreover, in relatively low environmental temperature, when a thick part of the tube was abutted onto the photoconductor, the charge gap became extremely large, which has caused failure in charging.
The present applicants therefore are in the process of developing a charging roller having a new configuration as follows. Specifically, the present applicants are in the process of developing a charging roller in which a cored bar as an axis member is coated with a discharging member composed of a conductive material, ring-like abutting members having a diameter larger than that of the discharging member are fixed on both end portions in the direction of the axis line of the axis member such that the discharging member is placed between these abutting members. In this configuration, the abutting members are not fixed to a relatively flexible discharging portion, which is made of a conductive rubber, but to a highly rigid axis member. For this reason, it is possible to avoid a situation in which the abutting members are embedded significantly in the discharging portion which became too flexible in high environmental temperature, and fluctuation of the charge gap which is caused by such penetration of the abutting members can be resolved. Moreover, the thick abutting members which are not deformed easily can be fixed firmly to the axis member by means of press-fitting, bonding, and laminating methods. Therefore, instead of obtaining an abutting member by attaching a thin film, which is deformed easily, to the discharging member and operating it as the abutting member, reduction of durability due to peeling of the abutting members can be prevented. Further, as the abutting members, it is possible to use the one such as rigid plastic formed into the shape of a ring, which is made of material in which thickness deviation can be adjusted more easily than the heat shrinkable tube. Therefore, toner adhesion and charge irregularity caused by the change in temperature and thickness deviation of the abutting members can also be prevented.
However, it has been discovered that, in such a charging roller as well, according to the material of the charging member or abutting member to be selected, fluctuation of the charge gap is caused by the change in temperature, thereby causing toner adhesion or charge irregularity.
As described above, in a conventional non-contact roller type, according to the set value of the charge gap or the material of the discharging portion of the charging roller, it was difficult to subject the latent image supporting body to charging evenly, causing charge irregularity.